We construct an algebra of
pseudodifferential operators on each groupoid in a class that generalizes differentiable
groupoids to allow manifolds with corners. We show that this construction
encompasses many examples. The subalgebra of regularizing operators is identified
with the smooth algebra of the groupoid, in the sense of non-commutative geometry.
Symbol calculus for our algebra lies in the Poisson algebra of functions on
the dual of the Lie algebroid of the groupoid. As applications, we give a
new proof of the Poincaré-Birkhoff-Witt theorem for Lie algebroids and a
concrete quantization of the Lie-Poisson structure on the dual A∗ of a Lie
algebroid.